It will be understood that it is important to accurately measure the clearance between various components in a machine such as a gas turbine engine. Previous non optical methods of achieving clearance measurement have utilised capacitance probes. This approach has difficulties with regard to access in that the necessary equipment is relatively large and it will be understood that there may be temperature limitations with regard to the construction of the capacitance probe. In addition, capacitance probes generally look end on to a part or onto an added part that may not always be available with regard to determining clearance measurements in a gas turbine engine.
With rotating components it is important to be able to note the axial as well as radial displacements for such situations as clearance measurement. It has been known to utilise optical devices for displacement measurement in terms of measuring the scatter of a reflected light beam dependent upon displacement between the emitter and the reflective surface, that is to say variations in displacement of that surface or chassis upon which the light emitting fibre or other conduit is placed. Unfortunately, previously such optical detectors have been limited in their ability to identify both axial and radial displacement.